PROTEINS

PROTEINS

Proteins are macromolecular organic substances formed from simple or complex chains of amino acids; they are present in the cells of all living organisms in the proportion of over 50% of the dry weight. All proteins are amino acid polymers, in which a gene encodes their sequence. Each protein has its unique amino acid sequence, determined by the nucleotide sequence of the gene.

Protein biosynthesis is a process by which each cell synthesizes its proteins through a process that includes many stages, synthesis starting with the transcription process, and ending with the translation process.

• transcription - the process of transcription requires the presence of a single, double-stranded DNA molecule, called "template" DNA, a unit that enters the "initiation" process. This is where the RNA polymerase enzyme acts, a protein that binds to a specific region of the DNA molecule, a part (called a promoter), where the transcription will start. As RNA polymerase binds to the promoter, the DNA chains will begin to disassemble. The next process in which DNA enters is the process of elongation (elongation of the chain). As RNA polymerase moves along the DNA strand, complementary ribonucleotide synthesis (mRNA - messenger RNA) occurs. This RNA, as its name implies, can also move to other parts of the cell, such as the endoplasmic reticulum or cytoplasm.

• translation - During translation, the mRNA transcribed from DNA is decoded by ribosomes for protein synthesis. This process is divided into three stages: initiation, elongation, terminal phase. The ribosome has binding sites that allow another tRNA molecule (transfer RNA) to bind to an mRNA molecule, a process accompanied by the presence of an antibody. As the ribosome migrates along the mRNA molecule (one codon once), another tRNA molecule is attached to the mRNA. The release of primary tRNA occurs, and the amino acid that is attached to it is bound to secondary tRNA, which links it to another amino acid molecule. The translation continues as the amino acid chain is formed. At one point, a stop codon appears, a sequence consisting of 3 nucleotides (UAG, UAA), signaling the end of the protein chain. Even after the translation is over, the protein chains may undergo post-translational modifications and the folding of the protein chain, responsible for the secondary and tertiary structure. Post-translational changes refer to the possibility of forming disulfide bonds, or of attaching to the protein skeleton different groups as biochemical role: acetate, phosphate, etc.

THE MAIN ROLES IN THE BODY

• plastic
• functional - realizes the oncotic pressure, participates in the acid-basic balance, participates in the formation of hormones and enzymes, constitutes membranous receptors, enters the constitution of other active substances
• defense - the restoration of the damaged tissues, antibodies, the trophicity of the cells of the defense systems, increases the resistance to the harmful action of some toxic substances.
• energetic - releases 4.1 kcal / g. They do not burn completely and result from their metabolism, indole, tryptamine, histamine.

TYPES OF PROTEINS

Depending on their chemical composition, they can be classified into:

HOLOPROTEINS with the following classes of proteins:

Globular proteins (spheroproteins) are usually water-soluble or saline substances:

• protamines - are small, arginine-rich nuclear proteins that replace late histones in the haploid phase of spermatogenesis and are considered essential for sperm head condensation and DNA stabilization.
• histones - are the main protein components of chromatin, representing the basis on which DNA is twisted, so if they did not exist, the non-spiralized DNA would be very long.
• prolamines - are a group of plant storage proteins that have high amino acid content. They are found in plants, mainly in cereal seeds such as wheat (gliadins), barley (barley), rye (secalin), corn (corn), sorghum (caffirine) and oats (avenin). They are high in glutamine and proline and have low water solubility.
• gluten - Gluten is a cumulative term, inexactly defined, for different protein components. It is of vegetable origin, present in cereals. The elasticity of a kneaded dough from cereal flour is given by gluten. The proteins that make up this gluten are responsible for the existence or non-existence of this dough property. This elasticity is present in the wheat dough. The quality and quantity of gluten in flour are the main characteristics on which its baking properties depend. Flour with higher gluten content is used for more top bakery products. The low gluten content gives the bakery products a smaller volume, flattened shape, and a little durability.
• globulins - are a family of globular proteins that have higher molecular weights than albumin and are insoluble in pure water, but dissolve in dilute salt solutions. Some globulins are produced in the liver, while the immune system makes others. Globulin, albumin, and fibrinogen are the main proteins in the blood. The average concentration of globulin in human blood is about 2.6-3.5 g / dL.
• albumin- is the generic name of any protein that is soluble in water, is partially soluble in concentrated saline, and is denatured by heat (coagulated). Substances that contain albumin, such as egg white, are called albuminoids. In mammals, albumin is synthesized by the liver. The inability to synthesize it is an essential manifestation of chronic liver disease. An example is serum albumin, which is found in blood plasma and is vital in maintaining plasma volume. Low serum albumin levels may indicate malnutrition.

Fibrillary proteins (scleroproteins) characteristic of the animal kingdom, with the role of support, protection, and mechanical resistance:

• keratin - proteins from the epidermis, hair, feathers, nails, hooves, and horns are distinguished by high sulfur content. Keratins are insoluble in both hot and cold water, as well as in saline solutions. Because of this, keratin has high inertia towards chemical agents as well as enzymes.
• fibroin - the fibrous component of the natural silk, is found in this material surrounded by an amorphous, sticky, sericin component, which represents approx. 30% of the total weight. In the two glands of silkworm, the proteins are contained in the form of a concentrated, viscous solution.
• collagen - is the main component of connective tissues, tendons, ligaments, cartilages, skin, bones, fish scales. There are numerous varieties of collagen. Collagen has a particular composition of keratin and fibroin, as it is rich in glycol, proline, and hydroxyproline does not contain cysteine and tryptophan. By prolonged heating with water, the collagen first soaks, then dissolves into gelatin or glue.
• elastin - constitutes fibrous tissue, with elasticity comparable to rubber, arteries, and the tendons, such as the tendon at the neck of the bull. Elastin does not turn into boiling gelatin with water and is digested by trypsin. Like collagen, elastin fibers are composed of pure amino acids, especially leucine, glycol, and proline.

HETEROPROTEINS are complex proteins that consist of a protein part and a prosthetic part:

Glycoproteins - are proteins that contain oligosaccharide chains (glycans) covalently attached to the side chains of amino acids. Glycoproteins are often crucial integral membrane proteins, where they play a role in cell interactions.

lipoproteins - are non-covalent aggregates (proteins) made up of lipids and proteins that form micelle-like particles. Plasma lipoprotein particles are used in all animal classes to transport water-insoluble lipids (fats), as well as cholesterol and cholesterol ester in the blood.

very low-density lipoproteins (VLDL) are formed exclusively in liver cells and transport mainly (stored and newly synthesized) triglycerides from liver cells to the rest of the body cells.

low-density lipoproteins (LDL) are formed in liver cells and mainly transport cholesterol or cholesterol esters from the liver to the rest of the body cells.

high-density lipoproteins (HDLs) are also produced in liver cells. Still, they mainly transport cholesterol from the rest of the body's cells back to the liver and are also capable of coupling with LDL particles, and some of them carry cholesterol. And cholesterol esters, but can change triglycerides with them.

nucleoproteins - are any proteins that are structurally associated with nucleic acids, either DNA or RNA. Typical nucleoproteins include ribosomes, nucleosomes, and viral nucleocapsid proteins.

STANDARD AMINO ACIDS

Chemically, proteins are heteropolymers consisting of 20 L-α amino acids (so-called standard amino acids, see table), in which carboxyl groups can be combined with amino groups forming peptide bonds and resulting peptide chains.

Standard amino acids have various properties, properties that are directly responsible for the three-dimensional structure of the protein, but also its features.

In the polypeptide chain, amino acids form peptide bonds by coupling the carboxyl group with an amino group. Once bound in the protein chain, the amino acid is "transformed" into a "residual" amino acid, and the carbon, nitrogen, hydrogen, and oxygen atoms involved in the bonds form the "skeleton" of the protein.

When:

• the protein chain is shaken with a carboxyl group bearing the name of carboxy-terminus (or C-terminus)
• the protein chain terminates with the amino group, becomes amino-terminus (N-terminus).